This invention relates to a method and apparatus for the beneficiating of ores and is particularly directed to a method and apparatus for classifying ores by size, shape and density while concurrently leaching values from said ores.
Current practice in leaching of gold and other precious metals from an ore involves crushing and grinding the ore to reduce its largest particle size to the point where intimate mixing of a leaching solution, such as sodium cyanide in water, with the particles will cause the gold or precious metal in the particles to dissolve into the solution from which the gold or precious metals are extracted at a later stage in a process plant.
In gold mills currently in use ore which has been diluted in water during the conventional grinding and classification process in grinding mills is thickened in a rake thickener or other device to about 50% solids by weight and then pumped as a slurry to a series of tanks where leaching agents are added. By keeping the slurry agitated with paddle mixers or other mechanical, pneumatic or hydraulic means, intimate mixing of the particles and leaching in solution takes place. Usually a series of tanks are used, one overflowing to the next, such that a leaching time of 12 to 72 hours takes place.
Recently, the high cost of processing low-grade gold and precious metals ores has led to cheaper methods of leaching such as "heap leaching" and "vat leaching".
Heap leaching involves placing ore as a layer on a pad on the ground or other prepared surface. A leaching agent is sprayed onto the layer of ore, after which it percolates through the pile, dissolving some of the gold or precious metals. The solution is then collected by drainage underneath and around the pile to a sump, where the solution may be recirculated to the top of the ore pile as often as is necessary to leach out the gold or precious metals, or pumped to a plant to have the gold or precious metals extracted from the solution.
Vat leaching is similar to heap leaching, except that the ore is sometimes ground and placed in a pit or like tank instead of a pile such that the ore is immersed for a more effective soaking to improve contact of the leaching solution with the ore.
Heap leaching or vat leaching is not nearly as efficient as conventional leaching in agitated tanks because the larger particle sizes and lack of mixing does not allow an intimate contact to be made between the leaching agent and the gold or precious metals. Leaching times are typically 10 to 15 days for vat leaching and weeks or months for heap leaching. In addition, both methods are batch processes, requiring adding and removing the ore from the leach area by mechanical means.
It is known to beneficiate ores by classifying solids in slurries using beds fluidized by a counter-current flow of a liquid or gas medium. Known methods, such as methods for separating bitumen from oil sands, use a continuous flow of rising fluid, usually water, counter-current to descending solids and effect a separation according to size, shape and density. The product recovered usually is substantially diluted by the volume of separating medium required and must be thickened.
It is also known to beneficiate ores by classifying the ore to separate values from the gangue, and continuously chemically leaching the said values from the ore.
The intermittent fluidization of particulated ore in a tank has been used to classify ore particles into strata according to size, shape and density to beneficiate values as disclosed in co-pending U.S. patent application Ser. No. 273,536, filed Nov. 21, 1988 whereby the intermittent fluidization creates a tendency in the stratified particles to move from the feed end of the tank to the discharge end of the tank. A valve system is utilized to recycle lixivium and to generate the high pressure force required for intermittent fluidization. The provision of increased feed pressure of the fluidizing leach solution by means of a reservoir tank having liquid under a substantially higher pressure maintained by a pump is also disclosed. The present invention is an improvement over the prior art as it avoids the need for such increased pressure through the utilization of a rotating drum, either with or without hydraulic or pneumatic means, which continuously fills and intermittently supplies the lixivium under a fluidizing pressure head.
The present invention obviates the need for a valve system by utilizing a continuous flow of leach solution.
The present invention enables particulate ore to be rinsed several times within the confines of the tank before the ore is discharged to a tailings discharge conveyor. Such rinsing enables recovery of values which would otherwise be lost if they remained affixed to the tailings. The present invention employs intermittent fluidizing of tailings with a rinse solution, pressure being supplied by the use of a rotatable drum mechanism.